1. Field of the Invention
This invention is the field of removing metal or organometal species from exhaust vapors. More particularly, the present invention is in the field of removal of tin- or organotin-halide species from exhaust vapors discharged during manufacturing, generally glass-coating, processes.
2. Description of the Prior Art
A number of industrial processes release metal vapors into the atmosphere; in order to protect the environment, limits have been established and continue to be revised as to the amount of metal which may be so released. This fact may require modification or even abandonment of inefficient or poorly controllable processes; alternatively, additional process steps to remove metal species from the exhaust gases may be necessary to allow continuation of the processes. Such well-known processes employ methods such as scrubbing the exhaust gases with water, employing filters with absorbing solids, e.g., carbon, incinerating the exhaust gases, and passing the gases through dust filters or electrostatic precipitators.
Glass surfaces, especially bottle surfaces, are coated with metal oxides, and specifically tin oxides by a number of well-known industrial processes, including chemical-vapor deposition (CVD). The tin-oxide coating serves as a base for other coatings, i.e. waxes, to assist in further processing of the bottles by providing scratch resistance and reduced coefficient of friction. This coating process allows weigh reduction in glass packaging over that otherwise obtainable, and makes it possible to run bottle-filling lines at very high speeds.
In the tin-oxide CVD coating process, alkyltin chlorides are evaporated in a coating hood and caused to contact the glass surface of the freshly formed bottle, where the temperature is in the vicinity of 600 degrees Celsius (.degree. C.). From 10 to about 50 percent of the tin chemical reacts on the bottle surface to form a thin coating of tin oxide. The remaining metal chemical, hydrochloric acid and other decomposition products are exhausted from the coating hood as gaseous products.
Emission standards in some countries do not permit the release of deleterious gases into the atmosphere, and require treatment to assure that emissions are not harmful. However, the act of cleaning may itself lead to another waste stream, unless the waste material is recovered in a usable form. Some methods are available to clean the exhaust gases from glass-coating processes which use tin chemicals.
In principle, gases can be treated with high-energy devices, i.e., venturi scrubbers, in which process the recirculating water is kept at a neutral pH through the addition of sodium hydroxide. The tin chemicals precipitate, and are removed by settling, filtering, centrifugation, or other separation process which may be difficult to administer. Unfortunately, the precipitate has a strong tendency to adhere to equipment surfaces, and due to its finely divided nature, is difficult to filter. This can cause serious problem in maintaining the system in operation. Moreover, the system requires sensitive pH control, and relatively high labor costs can result if plugging problem occur.
U.S. Pat. No. 3,957,448 teaches the addition of ammonia to the exhaust gases of glass-coating equipment; the ammonia reacts with hydrochloric acid and the tin-containing chemical to form powdery solids. These solids are then filtered out of the gas stream in a bag house. Recovery of the tin values from the filtrate, however, introduces complications resulting from the presence of ammonia; the replacement of filter media requires maintenance labor and difficult handling of toxic wastes, and can create yet another waste stream.
Larkin, in U.S. Pat. Nos. 4,144,262 and 4,130,673, and Lindner, in U.S. Pat. No. 4,530,857, discuss the treatment of newly formed and still-hot glass objects with a vapor of monobutyltin trichloride (MBTC) to form a protecting tin-oxide layer on these objects, and the concomitant removal of MBTC from the exhaust gases of that process. Electrostatic precipitators or other devices are recommended.
To solve the problem of waste-stream removal, another method known in the art is that of contacting the exhaust gases with an aqueous solution of a base such as sodium hydroxide. It has been found, however, that known exhaust-gas-treatment methods suffer a number of disadvantages; most methods do not attain the required low levels of metal to permit free emission of the cleaned exhaust gases into the ambient atmosphere. This is the case, for instance, if an electrostatic precipitator or condenser is used.
Scrubbing with caustic solution generates an aqueous metal oxide, the oxide containing a sludge, or mud, which needs to be treated again by filtration or centrifugation, thereby generating a new problem, that of disposal of waste water. This method is also expensive, and generally requires an uneconomical labor input.